Windows Server 2008 NLB uses only the Primary Cluster Network Address

In Windows Server 2008 NLB (Network Load Balancing) we display a Network Address for each Cluster IP address, but we only use the Primary IP Network Address when responding to ARP Requests.  In Windows Server 2003, we did not expose each individual Network Address when there were multiple cluster IP addresses.  The confusion comes in when someone is configuring their switch or router with the Network Address for each Cluster IP as it appears in the NLB configuration manager show below.



You will notice that for has a Network Address of 03-bf-ac-18-01-c9, and has a Network Address of 03-bf-ac-18-01-ca. Multicast NLB uses the prefix 03-bf plus the IP address converted to hexadecimal.


03-BF-172-24.-1-201 becomes 03-BF-AC-18-01-C9

03-BF-172-24.-1-202 becomes 03-BF-AC-18-01-CA

In order to support multicast you will often have to add a static ARP entry to the ARP table on the closest router or layer 3 switch (per KB197862).  In NLB you can add additional Cluster IP Addresses, but for configuration of those network devices, you will only use the Network Address of the Primary Cluster IP Address.  The primary IP Address is listed first as the example below shows.


If you configure hardware devices with the Network Address of each additional Cluster IP Address, you will see network connection failures for the Additional Cluster IP addresses when connecting from a different subnet.


Client can connect to the dedicated IP address and the Cluster Primary IP address when connecting from an IP address on a different Subnet, but attempting to connect to any of the additional IP addresses will fail.


If you configure the hardware ARP table with the Network Address of each individual Cluster IP address, the server will only respond to the Primary Cluster IP Network Address.


When you configure your hardware device’s ARP table, add each Cluster IP address with the Primary Cluster IP Network Address.

– Louis Hardy